Add a doc entry for setting up an OxCaml project with dune pkg

[Sudha247]

Closes #11652

Adds a tutorial for setting up an OxCaml project with Dune package management.

I contemplated where this should go, and thought this might be a good place, after all. It shows (1) how to setup an OxCaml project, (2) servers as a live example of the previous two tutorials.

#12111 is a more comprehensive tutorial about parametric libraries and OxCaml, but it doesn't specifically cover the topic of Dune package management with OxCaml. I recently ported an OxCaml project to build with Dune package management and the configuration was not as straightforward as I expected, and thought a tutorial on this topic might be useful.

This takes some material from https://github.com/gridbugs/hello-oxcaml (thanks @gridbugs!).

[Alizter]

Here are some screenshots:

I think having these drop-down code excerpts is kind of pointless. Perhaps it would be better just to have the code inline like we do elsewhere in the docs.

[Alizter]

The main title doesn't need to say Example: probably.

[shonfeder]

Glad to have this included!

[gridbugs]

I think we should be consistent with how we refer to dune package management in the docs. My preference would be to always write "Dune Package Management", or less formally to talk about "Dune's package management mechanism". The risk of referring to dune package management as dune pkg is that it looks like a command, and while that happens to be a command group for doing some package management tasks, conflating that with dune package management is incorrect (e.g. users of autolocking never need to run commands under dune pkg).

[Alizter]

At some point these how-to's turned into markdown rather than being rst documents which is slightly confusing. In rst we have syntax highlighting for dune files etc. Does anybody remember why markdown was chosen for these?

I would suggest writing this tutorial in rst, not only for consistency, but because it will play better with cross-linking that we have elsewhere in the docs.